Technical Abstract:
Seed glasses form during maturation drying and regulate seed longevity. Seeds continue to age within the glassy state and, even during cryogenic storage, viability eventually declines. Inevitability of aging suggests some level of molecular motion within the glassy matrix and quantifying these “relaxations” may provide insight about the mechanisms and kinetics of deterioration. Structural mechanics principles and Dynamic Mechanical Analysis (DMA) were used to identify and characterize molecular movement, and its converse structural stability, in seeds of varying water content between -130°C and 150°C. Glass transitions were identified as alpha relaxations, and additional beta and gamma relaxations were detected as peaks, plateaus or slope changes in elastic modulus (E’), loss modulus (E”), and damping factor (tan delta) measurements. Several relaxations were observed within the glassy state and are believed to reflect vibration and rotational motion of macromolecule side chains or molecular restructuring within the metastable glass. The temperature and size of alpha and beta relaxations were affected by seed water content and indicated that the plasticizing effects of water increased molecular mobility and the probability of physical aging in the glass matrix. Pea cotyledons were more structurally stable at temperatures below the beta transition and water contents between 0.03 and 0.07 gH2O/g dry mass, suggesting that these are the optimum conditions for maintaining viability. The response of mechanical properties to temperature changed as pea seeds were dried to water contents less than about 0.05 g H2O/g dry mass, and perhaps point to a destabilizing effect of excessive drying. Changes in mechanical properties were also detected in seeds as they deteriorated. Application of DMA to seeds may provide a useful tool to predict moisture and temperature effects on seed longevity and to noninvasively monitor the progress of aging during seed storage.